When jet aircraft fly through clouds of volcanic ash, quantities of particles infiltrate their engines, wherein the dust particles that are carried by the wind consist essentially of silicon dioxide, SiO2. Despite the rise in temperature of the combustion air, these particles do not cause a burden in the area of the air intake—the fan—or of the subsequent compression, that is to say in the compressor, but the turbine that is driven by the combustion gases experiences significant thermal loading in its typical temperature interval up to 2200° C. from melting silicon dioxide particles.
The basic materials from which turbine blades are made are typically special alloys on a basis of chromium and nickel that are particularly suitable for high temperature applications, and are cooled by an internal cooling system and/or external film cooling. However, the base material of the turbine blades is protected essentially by the fact that a thin ceramic surface coating having very low thermal conductivity and based on zirconium dioxide, ZrO2, with an addition of vanadium oxide, V2O5, is applied with a coating thickness of about 0.2 mm. This columnar insulating ceramic coating literally sucks itself full of the silicon dioxide present in the form of molten droplets. However, the SiO2 filling or saturation that is created thereby in the ceramic coating largely negates the very important thermal insulation effect of the ZrO2/V2O5 columns. The increased thermal load on the base material is associated with a significant risk of failure for the turbine blades, consequently for the engine and ultimately for the aircraft itself. Turbine blades that are contaminated in this way become unusable and a new coating must be applied to them. In the processes currently known, this involves completely removing the existing coating in a pickling process, and then again constructing a ceramic coating layer to replace the one that has been removed on top of the base material.
Accordingly, a process of the kind described in the introduction was disclosed in DE 600 15 251 T2, in which in a radical pickling process a dense ceramic coating in the form of a thermal barrier coating (TBC) is removed from the surface of an object containing at least one representative of the group Al, Ti, Cr, Zr and oxides thereof as a gas-phase compound by total removal of the coating on the surface of the object, and the coating is then reconstructed on the base material. In this known process, a hydrogen gas enriched with hydrogen fluoride is used as the reduction gas. In the other processes known from WO 2009/049637 A1, WO 2006/061338 A1 and DE 10 2005 032 685 B4 as well, the coating is stripped completely in this way, and the removed oxide layer is then built up on the base material again, and process gases enriched with halogen ions are used in all these known processes.